The properties required of the binder for a magnetic recording medium require precise synthesis of the binder materials. First and foremost of the properties required is the ability of the binder to maintain the extremely small magnetic particles in a fixed position which permits them to be magnetized, demagnetized and to impart a strong, modulated magnetic signal over long exposure to environmental and mechanical stresses. This ability requires physical properties of the binder such as tensile strength, flexibility with high modulus and surface energy properties which promote dispersion and wettability which, to some extent, are mutually exclusive. In order to achieve overall balanced performance, compromises are made which permit acceptable limits of performance while optimizing specific properties such as modulus or wettability. In magnetic "tape" construction, properties such as flexibility are optimized. In magnetic "disk" construction, hardness and durability can more easily be optimized because less stress is put on flexibility.
As magnetic media have become more sophisticated and as the magnetic particles which the binder is required to hold in rigid and intimate contact become extremely small with geometrical designs which enhance magnetic strength, the role of the binder becomes even more critical.
Binder formulation is further complicated by the fact that in order to make the storage of magnetic tape less bulky, the substrate, which is usually a highly oriented polyester film, has been made thinner and thinner in each succeeding generation of tape products. As a result, the polymeric binder, whose prime function is to maintain the magnetic particle in fixed position, is required to assume part of the function of the substrate, i.e., contribute to the mechanical properties of the magnetic media such as flexural modulus and tensile strength. This is so because with thicker substrates, the modulus and the tensile strength of the composite were provided by the substrate. In modern tapes with thinner substrates, the binder will substantially contribute to these properties of the composite.
To balance the rigidity needed of modern binders with the flexibility needed to respond to the requirements of recording mechanisms, hard or rigid polymers are combined with soft or compliant polymers to achieve optimum performance. Furthermore, when the binder polymeric segments are judiciously selected, they can be very fluid, readily coated and cured by crosslinking to the desired state.
U.S. Pat. No. 4,405,684 to Blumentritt et al. discloses a rigid magnetic recording medium having finely divided magnetic particles dispersed in a thermosetting resin binder which is comprised of a blocked isocyanate having at least three reactive sites per chain and an oligomer having at least two hydroxy reactive sites per polymer chain and a molecular weight of 200 to 800 per hydroxy site. The patent discloses the incorporation of a high functionality rigid polymer segment such as a low molecular weight styrene-allyl alcohol copolymer to improve the hardness of binders which are also comprised of hydroxyl-terminated polyesters. The object of the invention taught in the patent is to provide resin binder systems having long pot-life and capable of achieving strong adhesion to a rigid substrate (such as an aluminum disc) used in the recording medium and to that end, blocked isocyanates are disclosed as crosslinking agents of the binder. The binder resin systems taught cannot be used in conventional flexible media due to thermal distortion and/or degradation of the flexible support base upon exposure to the temperatures needed to activate a blocked isocyanate.
U.S. Pat. No. 4,407,901 to Miyatsuka et al. discloses a magnetic recording medium comprising a non-magnetic base bearing a layer mainly consisting of ferromagnetic particles and a binder wherein the magnetic layer contains a copolymer having a polar functional group and a degree of polymerization of not more than 100. Examples of polar functional groups are carboxyl groups, hydroxyl groups, phenolic-OH groups, and sulfonic acid groups. The carboxyl groups and phenolic-OH groups of the polymers disclosed in the only specific examples react with polyisocyanate crosslinking agents at ambient conditions only at undesirably slow rates.